The audio compact disc player has become one of the most popular pieces of consumer electronics equipment in use today. Compared to prior audio analog electronics, the digital signal of the compact disc player does not compound noise and distortion. In digital audio signals' quality of sound is not a function of the reading mechanism nor the media (i.e. compact disc). Digital performance parameters such as frequency response, linearity, and noise are generally only functions of the digital-to-analog converter (DAC). Other influences, however, that can effect the performance of the audio compact disc player and not necessarily the quality of sound include hardware defects and the quality of the recorded digital media or compact disc (CD).
The audio CD stores about 650-MB of audio data in a standardized format. The CD consists of several stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Typical CDs have a diameter of 120 mm and hold 74 minutes of audio with a player scanning speed of 1.2 m/s. They are 1.2 mm thick and are made of a polycarbonate-plastic coated with a much thinner layer of super purity aluminum layer protected by a film of lacquer. The lacquer film is typically printed or silk-screened with a label. The audio CD has a 15 mm hole in the center used by some form of clamp or clip device of the audio compact disc player to hold it in place and allow it to be rotated by a servo device or motor.
The information encoded on a CD is a spiral track of “pits” molded or imprinted into the top of the polycarbonate layer with the areas between the pits called “lands.” Each pit is approximately 125 nm deep by 500 nm wide, and varies from 850 nm to 3.5 μm long. The lands establish a time function for proper servo speed. The spacing between the tracks is 1.6 μm. To grasp the scale of the pits and lands of a CD, if the disc is enlarged to the size of a sport stadium, a pit would be approximately the size of a grain of sand. The spiral begins at the center of the disc and proceeds outwards to the edge, that allows different size formats (i.e. disk diameter).
The CD is conventionally read by focusing a 780 nm wavelength semiconductor laser of the player through the bottom of the polycarbonate layer. The difference in height between pits and lands is one quarter of the wavelength of the laser light, leading to a half-wavelength phase difference between the light reflected from a pit and from its surrounding land. The destructive interference thus reduces the intensity of the reflected light compared to when the laser is focused on just a land. By measuring this intensity with a photodiode, one is able to read the data from the CD. The pits and lands themselves do not represent the zeros and ones of binary data. Instead a change from pit to land or land to pit indicates a one, while no change indicates a zero. This in turn is decoded by reversing an Eight-to-Fourteen Modulation used in mastering or creating the CD, thus revealing the raw data stored on the CD.
The CD can be ruined by numerous defects including smudges, dirt and scratches. Because the pits are much closer to the label side of a CD, defects on the label are more prone to create out of focus playback, whereas defects such as scratches on the clear-side of the plastic CD can be repaired by refilling them with plastic of a similar index of refraction as the polycarbonate.
It is of little surprise that the advent of the audio CD player has been followed by commercially available audio CD burners or recorders. Such recorders utilize compact discs commonly referred to as “CD-Rs.” A CD-R is physically different than a mastered CD in that they are not imprinted and do not have actual pits and lands. Instead, a CD-R is injection molded with a “blank” data spiral. A photosensitive dye is then applied, and then the discs are metallized and lacquer coated. The write laser of the CD recorder changes the characteristics of the dye to allow the read laser of the audio CD player to see the data as it would a master CD. In-other-words, the audio CD player is fooled by thinking what is read are pits and falls but in actuality the player is reading changed reflective characteristics of a dye.
Unfortunately, the likelihood that an error will occur in the audio CD player when playing a CD-R is far greater than when playing an imprinted/master CD. Such errors are the result of physical defects on the CD-R when the consumer records a CD using one or any combination of the following: substandard CD media (CD-R or CD-RW), low quality recording software, low power recording laser diode, overburning, warped CD-R, poor motor drive, etc. In addition, but less likely, is the consumer experiencing an error eject of the player from physical defects on a professionally manufactured pressed CD-DA. Regardless, even a CD-DA may be damaged with excessive dirt and scratches causing ejection of the CD from the audio CD player and leaving the end user to wonder if its the CD or the audio CD player that is defective, and creating needless warranty returns of the audio CD players that the manufacturer must then address.
Not surprisingly, applications of the audio compact disc player extend into the automotive markets. Like many other markets, the automotive industry is greatly influenced by warranty returns. If an automotive audio CD player consistently skips audio tracks, mutes audio tracks, or ejects CDs without apparent cause, the consumer or end user is likely to return the vehicle for warranty repair even if the cause is not justified and/or caused by a faulty CD and not the automotive CD player.